Windows 8 store app Dropbox get access token and upload issue

We are using the following code and using OAuth1 and HMAC-SHA1 signing method.

var authentication = new Authentication(AppKey, AppSecret);
var token = await authentication.RequestToken("https://api.dropbox.com/1/oauth/request_token", WebAuthenticationBroker.GetCurrentApplicationCallbackUri().ToString());

if (token != null)
{
    var url = "https://www.dropbox.com/1/oauth/authorize?oauth_token=" + token.Key + "&oauth_callback=" + WebAuthenticationBroker.GetCurrentApplicationCallbackUri();
    System.Uri StartUri = new Uri(url);

    WebAuthenticationResult WebAuthenticationResult = await WebAuthenticationBroker.AuthenticateAsync(
                                            WebAuthenticationOptions.None,
                                            StartUri);
    if (WebAuthenticationResult.ResponseStatus == WebAuthenticationStatus.Success)
    {

        authentication.GetAccessToken("https://api.dropbox.com/1/oauth/access_token?oauth_token=" + token.Key, token.Key, token.Secret);
        Upload();
    }

public async static void Upload()
{
    var picker = new Windows.Storage.Pickers.FileOpenPicker();
    picker.FileTypeFilter.Add(".jpg");
    picker.FileTypeFilter.Add(".png");
    picker.FileTypeFilter.Add(".gif");
    picker.FileTypeFilter.Add(".doc");
    picker.FileTypeFilter.Add(".docx");
    picker.FileTypeFilter.Add(".xls");

    StorageFile attachmentFile = await picker.PickSingleFileAsync();
    byte[] imgBytes = await GetPhotoBytesAsync(attachmentFile);

    string apiKey ="yvynusucnu104zf" ;

    string consumerSecret = "<redacted>";
    string tokenKey = "vam1o365ldgp4wf7";

    TimeSpan SinceEpoch = (DateTime.Now - new DateTime(1970, 1, 1, 0, 0, 0, 0).ToLocalTime());
    Random Rand = new Random();

    Int32 Nonce = Rand.Next(1000000000);

    var uri = new Uri(new Uri("https://api-content.dropbox.com/1/"),
        String.Format("files_put/{0}/{1}",
        "dropbox", @"Photos\car-acc2.jpg"));

    var requestUri = SignRequest(uri, apiKey, consumerSecret, tokenKey, "PUT");
    HttpWebRequest Request = (HttpWebRequest)WebRequest.Create(requestUri);
    Request.Method = "PUT";

    //Request.Headers["Authorization"] = Data;



    using (Stream requestStream = await Request.GetRequestStreamAsync())
    {
        await requestStream.WriteAsync(imgBytes, 0, imgBytes.Length);
        await requestStream.FlushAsync();
    }

    HttpWebResponse Response = (HttpWebResponse)await Request.GetResponseAsync();
    StreamReader ResponseDataStream = new StreamReader(Response.GetResponseStream());

}